1. Field of the Invention
The invention relates to a multifocal lens for digital still cameras (hereinafter referred to as DSC), of which focal length can be changed from wide angle to telephoto and vice versa with the back and forth (up and down) movements of the second lens or lens group between the first and third lenses or lens groups.
2. Background Art
In DSCs, a zoom lens is usually used to obtain multiple focal lengths, and in most cases has three lens groups, namely the first lens group, the second lens group, and the third lens group. The first and second lens groups move simultaneously while the third lens group is fixed not to move.
The movements of the first and second groups of the lenses change the focal length in straight succession working like a zoom lens. From wide angle (shortest focal length of the lens) to telephoto (longest focal length of the lens), the two groups of the lenses must be moved correspondingly with each other. From one focal length position to another, the moving strokes of two groups of lenses are determined by the design of the lens. Usually, the moving strokes of these two groups of the lenses are controlled by cams mechanically, which are groove cams in most cases.
FIG. 1 shows a typical design of zoom lens with 2xc3x97 (two times) zooming ratio for DSCs. The first group of lenses (21) and the second group of lenses (22) move back and forth according to the change of the focal length and the third group of lenses (23) is fixed at the rearmost position and does not move, and the lines (24, 25) show the movements (tracks) of these two groups. The straight line (24) shows the movement (track) of the second group of lenses (22), and the curved line (5) shows the movement (track) of the first group of lenses (21). Each group moves at a time correspondingly with each other following the predetermined tracks from one focal length point to another. In FIG. 1, five focal length points are shown as examples of zooming positions. The position #1 shows the positions of the two groups of lenses at the longest focal length of telephoto position, #2 at the second- longest focal length position, #3 at the midpoint focal length position, #4 at the second- shortest focal length position, and #5 at the shortest focal length of wide angle position.
To enable the two groups of lenses to carry out the correspondent movements effectively, a mechanism with groove cams is usually used. FIG. 2 and FIG. 2-a show the conventional and mechanical controlling system of zoom lens, which has six groove cams. One group of the lenses is controlled by three groove cams of same curve and dimensions as a set provided on the periphery of the outermost lens barrel (a tube to hold the whole lens assembly), and another set of three groove cams of same curve and dimensions provided on the same periphery of the lens barrel controls the other group of the lenses.
The first set of the groove cams (26) provided on the outermost lens barrel (27) controls the first group of the lenses (21), and the second set of the groove cams (28) provided on the outermost lens barrel (27) controls the second group of the lenses (22). At the bottom rim of the outermost lens barrel (27), the gear teeth (29) are provided, and are engaged with the driving gears (30) of the stepping motor (31). When the zooming switch is turned on, the stepping motor (31) starts rotating, and then the outermost lens barrel (27) starts rotating. While the outermost lens barrel (27) rotates, the groove cams (26, 28) move, too, and the two groups of the lenses (21, 22) also move back and forth (up and down) by the movements of the pins (32) fixed on the lens barrel of each lens group and inserted into the slots of the groove cams, which move according to the curves of the groove cams (26, 28). The curves of the groove cams (26, 28) are designed according to the design data of the zoom lens accurately, namely according to the tracks like the tracks (24, 25) shown in FIG. 1. The direction of lens movement (back or forth) is determined by the direction of stepping motor rotation.
In this type of mechanical controlling of zooming, a high degree of accuracy of the groove cams is required and it is very difficult to manufacture lens barrels with such accurate groove cams, and consequently the cost of such accurate parts and components is quite expensive. The assembling and adjustment of the lens with such controlling mechanism are also quite difficult.
As explained so far, to fix the zoom lens at each focal length position accurately through zooming, the first and second groups of lenses must be operated quite accurately at a time correspondingly with each other, and the operation of a zoom lens like this mechanism must be very complicated and expensive.
On the other hand, users practically tend to jump from wide to telephoto or vice versa while skipping all the midpoints. Unless the difference of focal length between wide and telephoto is fairly big, it is difficult for the users to find any image effects of interests at the midpoints. This fact means that many of the users of DSC with zoom function do not fully use the expensive and complicated mechanism of zooming actually and practically, and simply use the shortest focal length position of wide angle and the longest focal length position of telephoto only.
To use the zoom lens only at its shortest or longest focal length positions, the expensive and complicated conventional groove cam mechanism can be omitted, and it is possible to have a much simpler mechanism with less and more inexpensive parts and components, and much easier way of assembling.
It is therefore an object of this invention to provide a zoom lens for a DSC in which the focal length can be instantly changed from the shortest length to the longest length and vice versa while skipping all the midpoint focal lengths.
It is another object of this invention to provide a zoom lens for a DSC eliminating the expensive groove cam zooming control, in which two groups of lenses move simultaneously.
As shown in FIG. 1, the track of the first group of lenses is curved very slightly, but the movement is relatively very small. Especially, the position difference of the first group of lenses between the telephoto position (the longest focal length, position #1) and the wide angle position (the shortest focal length, position #5) is extremely small, and the difference between the shortest and longest focal lengths of the first group of lenses could be considered non-existing. If users skip all the midpoint focal lengths where the first group of lenses moves rather largely back and forth (up and down) to change the focal lengths and to have the shortest or longest focal length position, it will be not necessary to move the first group of lenses at all.
Now to avoid the so far mentioned complicated two lens group controlling, by skipping the midpoint focal length positions, the first group of lenses can be fixed at a position, namely at the wide angle or at the telephoto position, because they are very close to each other and practically the same. Thus, by fixing the first group of lenses at the position, the groove cams for the first group of lenses can be omitted.
The second group of lenses moves linearly between the wide angle and telephoto positions. Its movement is quite straight and simple, yet to have this kind of lens movement, we need the groove cams. However, if the zooming jumps from wide to telephoto or from telephoto to wide without stopping at any of midpoint focal lengths, the groove cams for the second group of lenses can be also omitted, because it is not necessary to follow the zooming data of lens design to obtain the accurate position of each midpoint focal length.
In this invention the first group of lenses is fixed at the wide angle (or telephoto) position, and the first groove cams arc omitted, and then the mechanism moves the second group of lenses instantly from wide to telephoto without stopping at any midpoint focal lengths, and vice versa, so the second group cams are also omitted.
In the case of conventional film cameras (analogue cameras like 35 mm format film cameras), even though the optical designs of zoom lenses are similar to those of DSCs, their lenses are much bigger in size and heavier in weight in comparison with the DSCs, and it is quite difficult or practically impossible to use this direct jumping zoom method. Because the speed of movement of the second group of lenses cannot be so high due to its heavy weight and also long stroke of the second group of lenses movement, and while users are switching the focal length from telephoto to wide or vice versa, the image will be out of focus or soft in focus for a period of time, which is unacceptable to the users. However, in the case of the DSC, the second group of lenses can be moved quite instantly thanks to its very small size and light weight, and the soft images or images out of focus during jumping from wide to telephoto or from telephoto to wide can be practically negligible for the users, because such images appear only in a mere instant.
By moving only the second group of lenses between the two positions, the focal length can be changed from the wide angle to the telephoto or vice versa. The second group of lenses simply can jump up to the telephoto position or jump back to the wide-angle position, neglecting all the delicate positions of midpoint.
Thus, by fixing the first group of lenses at a position and by using a simple mechanism to move the second group of lenses jumping from the wide-angle position to the telephoto position or vice verse without stopping at any of midpoint focal length positions, the expensive and complicated mechanism with groove cams can be omitted from the zoom lens for the DSC, and the cost can be tremendously saved and the assembling of the zoom lens can be also far much easier than that of conventional groove cam zooming system.